This invention relates to a satellite communication system which adapts a Time Division Multiplexing(TDM) system using a static orbit communication satellite, and more particularly, to a guard time reducing system in data communication from an earth station to the static orbit communication satellite and a method thereof for realizing a guard time that each packet is near each other in time without overlapping each other, when each of a multiple of earth stations respectively transmits a packet data to the static orbit communication satellite then each packet lands on the satellite.
In general, in the satellite communication system using the TDM system, it is used to divide a time into small units and to transmit a part of data by using the divided small times at predetermined intervals for data communicating between the earth stations and the satellite. This method enables to alternatively arrange data transmission operation at time axis in a physical channel, thereby to set a multiple of individual channel.
In the general satellite communication system, respective earth station cannot exactly determine transmission time of packet data. This results in interference between the packet data because respective earth station does not serially transmit the packet data in time without overlapping. For the purpose of preventing the interference between the packet data, it is proposed to have sufficient idle time removing data collision between the packet data, to permit some collision despite of quality degradation of the packet data or to prevent the interference under control of a center station. These methods however, degrade the use efficiency of the channel.
A Network Access Unit(NAU) to transmit synchronous data and packet data with regard to the satellite communication system using the TDM system is disclosed in U.S. Pat. No.5,719,858. The NAU includes a network access module, synchronous application modules, packet application modules, and a packet manager. The network access module provides the interface between a TDM bus and a network facility. The synchronous application modules couple synchronous data equipment, e.g., telephone equipment , to the network access module via the TDM bus. The packet application modules couple packet data equipment, e.g. a data terminal to the packet manager. The later is a common resource module that performs integral aggregation in one direction, and distribution in the other direction, of packet stream between network access module and each packet application module, via the TDM bus and wideband packet buses, respectively.
As a reset of coupling the packet data to the TDM bus, packet manager provides a single multiplexed packet stream to the network access module for transmission across the network interface. Also, the NAU provides a TDM bus in conjunction with one or more packet buses which taken together provide more bandwidth than is required to support the network interface. This additional bandwidth is used to support multiple point-to-point packet connections. Packet manager not only aggregates the packet data, as mentioned above, but also allocates a fixed amount of the TDM bandwidth to the packet application modules.
Also, the instantaneous, or peak, data rate of all outbound packet streams taken together may be grater than the xe2x80x9cfixed amount of TDM bandwidthxe2x80x9d allocated for packet data on the network interface. These peak data rates create a large demand on both the overall packet bus capacity and on the packet handling requirements of packet manager. For example, once the packet application modules exceed their allocated network interface bandwidth, the packet manager must take steps to prevent the loss of any packet data. These steps include buffering the packet data, which may require a buffer of considerable size to support all of the packet application modules, and, perhaps, flow control to throttle the packet traffic.
The prior art, however, the complexity of the packet manager increases not only with the number of the packet application modules that the packet manager must support but also with the respective data rate requirements of these packet application modules.
Therefore, the present invention has been made in view of the problems, and it is an object of this invention to provide a guard time reducing system in data communication from the earth station to the static orbit communication satellite and a method thereof to maximize the use efficiency of the communication line.
In accordance with one aspect of the invention, there is provided a guard time reducing system in data communication from a earth station to a static orbit communication satellite characterized by: a communication unit for generating, storing, transmitting/receiving, and operating of data; a Global Positioning System(GPS) timing unit for receiving position information of the static orbit communication satellite from the communication unit, computing the position of the earth station, finding propagation passed time from the earth station to the static orbit communication satellite and then outputting a time clock and a frequency as fast as the propagation passed time than Universal Coordinated Time(UTC); a packetizing unit for receiving data through the communication unit and diving the data to match a time frame, receiving the time clock and the frequency from the GPS timing unit and transmitting a packet synchronized with the time clock and the frequency to the static orbit communication satellite.
In accordance with other aspect of the invention, there is provided a guard time reducing method in data communication from a earth station to a static orbit communication satellite characterized by the following steps of: finding , in the GPS timing unit, a propagation passed time from the static orbit communication satellite to the earth station; and transmitting a packet as fast as the propagation passed time than the UTC when a packetizing unit transmits the packet to the static orbit communication satellite.